1. Field of the Invention
This invention relates to a thermal head used for a thermosensitive printer, thermal transfer printer, etc.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional thermal head. In the thermal head of this example, a glass glazed layer 2 is formed partly on an insulating substrate 1 made of alumina, a heat generating resistor layer 4 and a power supply conductor layer 5 are sequentially laminated through an undercoating layer 3 on the glazed layer 2 so that the power supply conductor layer 5 is patterned in an individual electrode 5a and a common electrode 5b. The heat generating resistor layer 4 is exposed in a gap between the individual electrode 5a and the common electrode 5b to form a heat generating portion A. The surface of this thermal head is protected by an overcoating layer 6.
The overcoating layer 6 prevents the heat generating resistor layer 4 from being oxidized and the heat generating portion A from being damaged.
Heretofore, it has been proposed to use as the material for forming the overcoating layer 6 a silicon-aluminum-oxygen-nitrogen compound (hereinafter referred to as "a sialon").
The sialon has 9 to 10 of high Mohs hardness, excellent wear resistance, and a film formed of the sialon has an oxidation preventiveness. Thus, the overcoating layer 6 may be formed only of the sialon film to reduce the thickness of the overcoating layer 6. Further, the sialon has excellent thermal conductivity. Thus, the thermal head in which the overcoating layer 6 is formed of the sialon has good thermal responsiveness. In addition, since the sialon film has excellent thermal impact resistance, the thermal head can cope with high speed printing.
However, such a thermal head has poor bondability of the heat generating resistor layer 4 and the overcoating layer 6 in the heating portion A due to fusion-bonding resistance of the sialon; and if a hard foreign material causes the overcoating layer 6 to crack in case of printing, the overcoating layer 6 is exfoliated from the cracked portion, the adjacent heat generating resistor layer 4 is rapidly oxidized, which increases the resistance value. As a result, the heat generation amount is markedly reduced, to such an extent that printing becomes impossible. Thus, the printing life of such a thermal head is limited.